The Synchronous Optical Network (SONET) is the transport technology of choice for high bandwidth communication across a Wide Area Network (WAN). The SONET standard defines a multiplexing hierarchy for digital communications, including transmission rates, signals and interfaces for fibre optic transmission. SONET also provides a standard synchronous optical transmission protocol. The broadband fibre network it supports is formed by a family of network elements conforming to the SONET interface requirements.
The SONET protocol implements communication signals which are multiplexed in increments of approximately 51.8 Mbps. The 51.8 Mbps service is the finest granularity SONET service and is referred to as STS-1. Higher bandwidth SONET services are in integer multiples of STS-1. The most popular are STS-3 (155 Mbps), STS-12 (622 Mbps), STS-48 (2.48 Gbps), STS-192 (10 Gbps). In the optical domain these services are referred to as OC-3, OC-12, OC-48 and OC-192 respectively.
SONET communication signals are channelized. Under management control, a SONET STS-1 stream can be time divided into payload channels of down to DS0 (64 Kbps) granularity. Multiple DS0 channels can be concatenated to enable the carriage of higher bandwidth service such as T1 or T3. All channels in a SONET stream can be concatenated to enable the carriage of continuous data up to a rate equal to the rate of the SONET stream (less SONET transmission overhead). A 155 Mbps SONET stream concatenated in such a way is referred to as OC-3c. Concatenated streams may be multiplexed into higher rate SONET streams. For instance an OC-12 stream may be composed of four OC-3c streams.
The most widely deployed SONET topology is a dual ring linking a number of network elements (NEs). Each ring is composed of point-to-point links with adjacent (link partner) NEs in the ring. On a SONET ring, a service is provisioned as a bi-directional communication channel between two NEs on the ring. The communication channel can consist of, for example, a single DS0. In this case the NE performs an add/drop multiplexing function on this channel, receiving the data in the channel from the ring (drop) and replacing this data with the data to be transmitted to the link partner NE for this channel (add). The ring bandwidth allocation is static, in that a channel allocated to communication between two NEs cannot be used for any other purpose.
The dual ring topology is employed for media redundancy purposes. There are two widely deployed redundancy mechanisms: Unidirectional Path Switched Ring (UPSR); and Bi-directional Line Switched Ring (BLSR). In UPSR a transmitting NE puts a copy of the data on each of the rings and the receiving NE chooses which signal to receive. In BLSR all NEs use one of the rings for payload transport, leaving the other ring for redundancy. If the payload-carrying ring is broken, the NEs switch to the other “redundant” ring for continued payload transport.
Dense Wavelength Division Multiplexing (DWDM) is an enhancement of the SONET service, which uses optical techniques to put multiple streams onto a single fiber. Each stream on the fiber uses a different wavelength. This technique enables a significant increase in the bandwidth available using existing fibre links. Thus DWDM is particularly useful on congested fiber links, because it is frequently more cost-effective to invest in equipment to increase the throughput of the existing infrastructure rather than laying new fiber runs. DWDM can be readily implemented on existing SONET rings, including UPSR and BLSR topologies, by use of appropriate Layer 1 hardware/firmware in the NEs. Thus it is expected that logical ring WAN topologies (e.g. BLSR) will be popular for DWDM networks for the same reasons as SONET: redundancy and Metropolitan Area Network (MAN) fiber routing simplicity.
A Dual Counter Rotating Ring (DCRR), such as, for example, a Fiber Distributed Data Interface (FDDI) network, is a further enhancement of the SONET service, in which all payload traffic flows in one direction around one ring (the “payload ring”). Payload data frames are STRIPed by the sending node when they complete a round trip. In the event of a failure of a network component (e.g. a link on the payload ring, or a node) the nodes on opposite sides of the break redirect traffic onto the redundant ring, in which the traffic flows in the direction opposite that of the payload ring, to thereby close the ring.
The above-described existing WAN infrastructures are effective in permitting reliable communications for high bandwidth applications. However, there are a number of deficiencies of those prior art systems which limit utilisation of the bandwidth capacity of the installed fiber media. In particular, the SONET protocol prescribes static allocation of ring bandwidth, so that a channel allocated to communication between two NEs cannot be used for any other purpose, even if that channel is idle (i.e. is not carrying payload traffic). Additionally, one of the rings is effectively redundant, so that when the WAN is fully operational (which is the majority of the time) only half of the total bandwidth capacity is actually used for payload traffic. Finally, in the event of a failure of a network component (e.g. a link or an NE), all of the operational NEs on the WAN must be informed of the existence of the failure, and then must switch over to the redundant ring. As the number of NEs on the WAN increases, the time required to accomplish this operation (the “fail-over time”) also increases.
Accordingly, there remains a need for a network control system, usable in conjunction with dual-ring SONET topologies, which permits a more efficient utilisation of the total bandwidth capacity of the installed fiber media.